I have an untraceable heavy knock on my 3rd Gen. It happens when manouvering at low speed, typically when backing into a parking space and turning the steering / moving from forward motion into reverse. It's a depp thud, which sounds like it's going through the chassis. I don't feel it through the steering. It is not coming from the UJs or diff as it does not happen when I take up the drivetrain slack with the clutch (it has a manual transmission). The vehicle is not in 4WD.

It doesn't happen every time, but often enough to annoy me. I keep the vehicle in fine condition and have rebuilt most of the drivetrain over the last few years. Here are a list of things I am 99% sure are NOT causing the noise:

I'm pretty stumped as to what could be causing this noise. It never happens whilst moving at speed, only at low-speed manouvering.

I'm not with the vehicle right now, but the only thing I can think of is that the transmission mount looked to have 'collapsed', i.e. the top part (which bolts to the transmission case) is sitting on the lower part, even though the rubber looks intact. Could this cause a heavy thumping sound??